I'm in the process of painting the rear end, and other undercarriage pieces with single-stage urethane. I've been following the directions given me, to use 8 parts paint, 4 parts reducer, and 1 part hardener. OK so far. I painted a few pieces 3 days ago, and although they seem throughly dry, I can scratch it with a fingernail. It's been sunny, with temps in the 70's. I know it will never be as hard as the powder coating on the frame, but it seems to me that it ought to be harder than spray can paint. How hard can I expect this paint to get? And, how much time should I give it before working with the items to put them back together? Thanks for any advice you can offer